1. Field of the Invention
This invention relates to centralizing devices and to such devices which can additionally be used as down-hole position locators and also when used in tandem to define an axial alignment.
2. Summary of the Prior Art
In down-well operations in the oil and gas production and exploration industries, it is frequently a requirement that a tool, such as position sensors, shall be accurately centralized within the casing pipe or other cylindrical member and this has become even more important as position sensors or logging devices per se have become more and more accurate. Such high accuracy in the sensor itself is of little value if the precise location relative to the casing pipe or other cylindrical member becomes indeterminate because of non-central location of the sensor within the casing pipe.
Again, maintained precision of centralization becomes far more important where logging activities may involve directions ranging from the vertical substantially to the horizontal in the same casing pipe, and presently known devices for maintaining centralization within a casing pipe are not successful in achieving their objective within horizontal bore holes because of the gravity effect. This becomes an even greater problem when the sensing units have considerable length and weigh sometimes over 100 kg. It follows that the amount of support required for such sensing units as a horizontal orientation is approached can be substantial and it is desirable that appropriate support will be provided irrespective of the angular location of the sensing unit at any given location. Clearly, the strength under bending loads becomes more significant under conditions other than truly vertical.
The centralizing device must be able to operate effectively irrespective, within wide limits, of the diameter of the cylindrical structure in which it is working.
Centralizing devices have been proposed for example in U.S. Pat. No. 1,898,074 in which pairs of two-armed linkages are mounted externally of a central elongate member with a mounting assembly at each inner end and means biasing one mounting assembly so that the outer ends of the arms of each two armed linkage are, in turn, biased outwardly so that the outer ends of the arms contact the bore-hole or casing wall. This prior proposal is not, however, satisfactory as the strength is inadequate, more especially when the centralizer must be of slender construction so that it can be used in bore holes or tubing of small internal diameter, say 21/4 inches (6 cm). This problem of inadequate bending strength is magnified when, as is usually the case, the centralizers are used in tandem so that the overall length of the whole, including a sensing instrument between them, may be as much as 30 feet (10 meters) and, furthermore the assembly is used in a non-vertical orientation.
U.S. Pat. No. 4,549,630 to Graydon L Brown issued Oct. 29, 1985 discloses a device superficially similar to a centralizer, but the purpose is quite different, namely the transmission of shear wave energy to a bore hole. Strength is not therefore a primary requirement and there is no requirement for this device to operate in tandem with another.
U.S. Pat. No. 4,619,322 to Richard A Armell issued Oct. 28, 1986 discloses a centralizer including a central elongate body member supporting a plurality of two-armed linkages mounted inwardly on slide blocks movable on the central member. However the slide blocks lie outside of the elongate body member and, particularly for small diameter centralizers the bending strength is inherently not as high as desirable.
U.S. Pat. No. 3,789,511 to Groom et al issued Feb. 5, 1974 relates to a device for measuring the curvature of a pipeline. Once again a central elongate member is provided together with spring-loaded two-arm linkages, but the arms are mounted entirely externally to the elongate member so that for any given diameter of the latter the strength in bending is not as high as is necessary for applications contemplated by the present invention.
U.S. Pat. No. 2,899,633 to O. R. Smith et al issued Apr. 5, 1956 has some similarities to a centralizer, but there is no requirement to centralize within a borehole and no requirement for tandem operation. It is not apparent that any attempt has been made to provide a construction with high bending strength.
One object of the invention is to provide a centralizing device which can more readily be used within casings and tubings of differing inner diameters and which has adequate bending strength even when the bore hole diameter requires a centralizer of very small size.
In addition to centralizing, there is also a requirement for certain down-hole operations to establish substantially exactly an axial alignment within the tubing or casing and it is therefore another object of the present invention to provide an assembly which will define such a substantially exact axial alignment down-hole.
It is a further object of the invention to provide a bore hole centralizer in which spring-loaded arms of two-armed linkages exert a constant pressure against the bore-hole wall irrespective, substantially, of the angular configuration of the arms in any bore-hole over a range of diameters.